BANGKOK, Thailand -- Merrill Lynch & Co. said it will pay up to $68 million to take majority control of Phatra Securities Co., placing a bet on an eventual turnaround in the Thai stock market.

In acquiring a 51% stake in the large Thai brokerage firm, Merrill Lynch is taking advantage of Thailand's economic crisis to buy into a market that had been closed to foreign firms until financial turmoil struck last year. Foreign brokers...